(1) Field of the Invention
This invention relates to hydraulics, and more particularly to transfer valve systems for aircraft hydraulics
(2) Description of the Related Art
In an aircraft, a transfer valve (also known as a switching valve) is used to direct pressure and flow from one of two available hydraulic supply systems to a downstream circuit. The valve switches between supply systems based on the relative magnitudes of the system pressures and a pressure bias built into the valve. The bias is often achieved by providing different pilot areas on each end of the valve, the larger of the pilot areas being on the end acted upon by the preferred, or primary, system. A spring is also sometimes used to provide a bias. With both supply pressures at normal operating pressure, the forces acting on the valve are such that the preferred (primary) system is connected to the circuit and the other (secondary/back-up) system is blocked. The purpose of a transfer valve is to allow the back-up system to take over for a failed or failing primary system. The transfer valve also isolates one hydraulic supply system from the other. As the primary system pressure decays, the force balance on the valve favors the primary system less and the back-up system more, until eventually the valve shuttles from the initial primary position to the back-up position. In the back-up position the valve connects the back-up system to the circuit and blocks the primary system.
Once pressures reach the switchover condition, the transfer valve advantageously shuttles with a snapping action, without interruption, hesitation, or instability. The snapping action is advantageous for two reasons. First, it minimizes transient pressure spikes as the valve closes off flow from one system and opens flow from the other. Second, it prevents the forces acting on the valve from achieving equilibrium when the valve is mid travel. If the valve stops mid travel it could block both supply systems and create a hydraulic lock in the downstream circuit. This snapping action can be enhanced through the use of a mechanical, hydraulic or electric detent mechanism.
Two well known styles of transfer valve are solenoid-operated and pressure-operated valves. Solenoid-operated transfer valves provide a high degree of control. The transfer valve can be switched from one system to the other by energizing and dc-energizing the solenoid. Because the solenoid controls when the valve shuttles, this design does not require a detent mechanism. This approach is generally relatively complicated and requires a suitable electronic computer or system to control the solenoid. In addition to the computer system, some means of sensing pressure is required. Because this approach is more complicated, it requires a higher level of redundancy and is generally more expensive.
Pressure-operated valves shuttle based on the relative pressures of the two supply systems and the pressure bias and detent action designed into the valve. This style or transfer valve may require no inputs from other devices. Pressure-operated transfer valves may make use of a hydraulic or mechanical detent to achieve the desired valve behavior. With a mechanical detent, a mechanism holds the valve in position until sufficient shuttling force is developed to overcome the detent. Once the detent is overcome, the valve moves away from the detent and the detent force no longer acts to hold the valve in position. With the detent force suddenly removed, the forces acting on the valve are no longer in equilibrium and the valve shuttles with a deliberate motion. Mechanical detents are typically spring-loaded devices. The mechanical detent may introduce friction and side loads on the valve spool, both of which have a negative effect on the valve""s switching characteristic.
Accordingly, one aspect of the invention involves a hydraulic transfer valve system for coupling one of primary and secondary hydraulic systems to a hydraulic load. The hydraulic systems each have a source and a return and the load has an input and a return. The system has first and second valves and a passageway coupling the first valve to the second valve and having first and second ports at the first and second valves, respectively. The first valve has a first condition in which it provides communication between the primary hydraulic system source and the hydraulic load input, provides communication between the primary hydraulic system return and the hydraulic load return, blocks communication between the secondary hydraulic system source and the hydraulic load input, provides communication between the secondary hydraulic system return and the hydraulic load return, and blocks the passageway first port. The first valve has a second condition in which it provides communication between the secondary hydraulic system source and the hydraulic load input, provides communication between the secondary hydraulic system return and the hydraulic load return, blocks communication between the primary hydraulic system source and the hydraulic load input, provides communication between the primary hydraulic system return and the hydraulic load return, and does not block the passageway first port. The second valve has a first condition in which it blocks the passageway second port and a second condition in which it does not block the passageway second port and herein, with the first valve in its second condition, the passageway permits a flow from the primary hydraulic system source to the primary hydraulic system return.
In various implementations the valves may be sliding spool valves. The valves may be spring-biased in a direction from their second conditions to their first conditions. Pressure from the primary hydraulic system source may bias the valves in the directions from their second conditions to their first conditions. Pressure from the secondary hydraulic system source may bias the valves in directions from their first conditions to their second conditions. With the valves in their first conditions, a recirculating flow from the secondary source to the secondary return may be permitted through the second valve. A pressure sensor may be coupled to the second valve and output a signal indicative of a pressure in the secondary hydraulic system.
Another aspect of the invention involves a hydraulic transfer valve system with means for piloting a first valve so that with primary and secondary hydraulic systems initially operating normally and the first valve in its first condition, a decrease in a pressure of the primary hydraulic system source relative to the secondary hydraulic system source causes the first valve to toggle to its second condition. With the secondary hydraulic system initially operating normally and the primary hydraulic system source initially operating with an insufficient pressure and the first valve in its second condition, a sufficient increase in the pressure of the primary hydraulic system source relative to the secondary hydraulic system source causes the first valve to toggle to the first condition.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.